This invention relates to an FM audio signal receiver (tuner) for FM audio signals such as FM broadcast and television broadcast signals and, more particularly, to an FM audio signal receiver capable of improving characteristics of input FM audio signals in accordance with a state of the input FM signals.
In receiving broadcast wave such as FM broadcast wave and FM audio signals of television broadcast wave, conventional stereophonic audio receivers mounted on automobiles control the level, separation characteristic and frequency characteristic of input FM audio signals in accordance with field strength of these signals to change characteristics of the signals over the range from a weak field strength to a strong field strength and thereby obtain a tone which is more pleasant for listening.
As to the level, for example, a noise level tends to increase when an input signal is of a weak field strength. For preventing this phenomenon, a control called SMC (soft mute control) is performed in such a manner that, as shown in FIG. 2, the output level is decreased to decrease the noise level when the input signal is of a weak field strength so that an unpleasant noise can be reduced.
As to separation characteristic, noise tends to increase if the degree of separation is high when an input stereophonic signal is of a weak field strength. For preventing this phenomenon, a control called SNC (stereophonic noise control) is performed in such a manner that, as shown in FIG. 3, the degree of separation is decreased when the input stereophonic signal is of a weak field strength.
As to frequency characteristic, a noise in the high frequency region tends to become particularly unpleasant when an input signal is of a weak field strength. For preventing this phenomenon, a control called HCC (high cut control) is performed in such a manner that, as shown in FIG. 4, the high frequency region is attenuated when the input signal is of a weak field strength.
FIG. 5 shows a conventional receiver having functions of the above described controls of SMC, SNC and HCC. In the receiver of FIG. 5, an FM broadcast signal received by an antenna 10 is supplied to an intermediate frequency amplification and FM demodulation circuit 14 through an FM front end section 13 and a signal of a desired broadcasting station is demodulated. The demodulated signal is separated to left and right channel signals by an FM multiplex circuit 16 and delivered out for propagation.
In the receiver of FIG. 5, the respective controls of SMC, SNC and HCC are performed by supplying field strength outputs derived from a signal meter output of the FM demodulation circuit 14 to respective control terminals through semifixed resistors 24, 26 and 28.
For eliminating pulse noise such as an engine noise which occurs during receiving of an FM broadcast signal in a conventional stereophonic receiver mounted on an automobile, an FM noise cancellet 30 (e.g., chip No. LA 2110) is provided between the intermediate frequency amplification and FM demodulation circuit 14 and the FM multiplex circuit 16 as shown in FIG. 6. In this noise cancellet 30, as shown in FIG. 7, an FM demodulator output signal is applied to a high-pass filter 32 to extract a noise component in its high frequency component (over about 100 kHz). The extracted noise component is flattened by an averaging circuit 34 and the flattened noise component is compared with the unprocessed noise component by a comparator 36 to detect a pulse noise. A monostable multivibrator 38 is triggered at rising of the pulse noise to drive a gate circuit 40 whereby noise in the FM demodulator output signal is eliminated.
In a case where an FM broadcast signal is received by receiver mounted on an running automobile, a multipath noise is generated due to receiving both a direct wave from a broadcasting antenna and a reflected wave produced due to geographical and architectural situations in which the running automobile is located (i.e., multipath reflection interference). This noise deteriorates the quality of a reproduced tone so that it is very unpleasant to the listener. An FM broadcast signal is normally constant in its amplitude but its amplitude and phase are subjected to variation due to the interference when a reflected wave is added to a direct wave. This situation differs depending upon the ratio, delay time and phase difference between the direct wave and reflected wave. For preventing this multipath noise, in a conventional receiver, an output signal from an IF amplifier 14a before being applied to an IF limiter 14b is applied to an envelope detection circuit 42 as shown in FIG. 8 for detection of an envelope of the signal and therefore variation in the amplitude (in some cases, delay time and phase are also detected together with output frequency deviation of an FM demodulation circuit 14c). In accordance with the detected envelope, a control circuit 44 performs a control in the FM multiplex circuit 16 so that frequency characteristic of a reproduced signal is modified (mainly attenuation of the high frequency region) and separation characteristic is changed and, sometimes, the operation mode is changed from a stereophonic operation to a monaural (monophonic) operation. Since the receiving situation momently changes in the running automobile, timing of the control (i.e., rising time and duration of a control operation) is very important.
Since the receiver shown in FIG. 5 is constructed of an analog integrated circuit and discrete parts and the field strength outputs are applied to the respective control terminals through the semi-fixed resistors 24, 26 and 28 to perform the controls of SMC, SNC and HCC in analog manner, dispersion tends to occur in control characteristics to such a degree that it sometimes becomes necessary to make adjustments. Besides, respective control characteristics interfere with one another (i.e., adjustment of one parameter brings about change in another parameter) so that it is difficult to obtain a desirable setting state.
There are wide ranges of pulse amplitude and pulse width in a pulse noise such as an engine noise applied to the receiver. According to the noise canceller 30 shown in FIG. 7, however, noise having a pulse width which is determined by the monostable multivibrator 38 only can be removed and the noise canceller 30 is ineffective for a pulse noise having a larger pulse width than that. Besides, if input field strength is weak, a large amount of noise component is contained in the FM signal so that the high frequency component for detecting a pulse noise is instable with resulting deterioration in the accuracy of detection or, conversely, with a result that a pulse noise is erroneously detected due to an FM weak field strength noise leading to an erroneous control operation. For these reasons, setting for the control operation is extremely difficult.
It is also difficult for the construction shown in FIG. 8 to eliminate a multipath noise in all situations even if the contents of control are set in accordance with one situation in which a multipath noise occurs, because the situation in which the multipath noise occurs is very complicated. It is therefore difficult to perform a fine control depending upon the situation in which a multipath noise occurs. It is also difficult to match a rising time and duration of operation of the control circuit 44 in accordance with the situation of the multipath noise.
It is, therefore, an object of the invention to provide a receiver which is capable of improving characteristics of input FM audio signals in accordance with the state of the input FM signals.